1. Field of the Invention
The present invention relates to a lid of a battery container and a method of manufacturing the same. More specifically, the present invention is directed to an improvement in geometry and structure of a curled portion of a lid and a main body of a battery container largely influencing leakage of an electrolyte and also to an improvement in a method for manufacturing such lid.
2. Description of the Prior Art
FIG. 1 is a front view, partially in section, showing a conventional silver oxide battery for explaining the prior art of interest to the present invention. FIGS. 2 to 8 are end views of mid-sectioned part showing in succession different steps of a method of manufacturing the lid used in the FIG. 1 battery.
The silver oxide battery shown is a so-called button type battery which has been widely used as a power supply for an electronic wristwatch, for example. The battery container comprises an cathode container element 1 constituting a main body of the battery container and a anode container element constituting a lid of the battery container. The cathode container element 1 or the main body is made of an iron plate nickel plated and an cathode material 2, a separator 3 and a anode material 4 are in succession placed in a laminated fashion in the cathode container 1. The anode container element 5 or the lid is then placed on the anode material 4. The anode container element 5 is made of stainless steel and a copper layer is formed on the inner surface thereof, while the outer surface thereof is formed with a nickel layer.
The anode container element 5 is formed with a periphery portion 6 U letter shaped in section. A packing 7 is disposed substantially outside of the peripheral portion 6 and, when the opening edge of the anode container element 1 is curled inside, the same is engaged with the peripheral portion 6 of the cathode container element 5 with the packing 7 therebetween. As a result, a sealing structure for preventing a leakage phenomenon of an electrolyte is achieved.
Since the above described anode container element 5 has the peripheral portion 6 folded in a U letter shape in section, the periphery of the anode container element has an increased strength to buckling. However, since the hardness of a portion being curled largely contributes to prevention of leakage of an electrolyte, a much more increased hardness is desired.
An insufficient hardness of the peripheral portion 6 of the conventional anode container element 5 which is engaged with the opening edge of the cathode container element 1 is presumably caused by a conventional manufacturing process to be set forth in the following. According to a conventional manufacturing process, a blank 8 is cut to a desired shape and size is prepared as shown in FIG. 2. Then the blank 8 is subjected to a pressing process by means of an upper drawing die 9 and a lower drawing die 10, whereby the blank 8 undergoes a first drawing process. At that time, the outer peripheral portion of the blank 8 is strongly sandwitched to prevent from wrinkling, so that the thickness of the outer peripheral portion is decreased. Then the blank 8 is subjected to a second drawing process by means of an upper drawing die 11 and a lower drawing die 12, as shown in FIG. 4. Again in this step the thickness of the outer peripheral portion of a material being worked 13 is decreased. Then the material being worked 13 is subjected to cutting to be trimmed to a desired size in diameter, as shown in FIG. 5. Thereafter, as shown in FIG. 6, the flange portion 14 at the outer periphery of the material being processed 13 is slightly bent preparatively upward as viewed. Then, as shown in FIG. 7, the flange portion 14 of the material being worked 13 is fully bent so that the periphery of the material being worked 13 may be U letter shaped in section. Then ultimately, as shown in FIG. 8, the flange portion 14 is further pressed, whereby the flange portion 14 is brought to be closer to the main body portion of the material 13. Thus the anode container element 5 as shown in FIG. 8 is obtained and the flange portion 14 constitutes a portion of the abovedescribed peripheral portion 6 U letter shaped in section.
It has been observed that according to such conventional process the steps as shown in FIGS. 3 and 4 and 6 to 8 involve some problems. This will be described in more detail with reference to FIG. 9, which shows in an enlarged manner the peripheral portion of the lid.
A first problem is that the thickness of the outer peripheral portion of the blank 8 or the material being worked 13 is decreased to be made thin at the steps shown in FIGS. 3 and 4. The outer peripheral portion thereof is ultimately brought to the position of the outer peripheral side 6a of the peripheral portion 6 U letter shaped in section. Accordingly, the outer peripheral side 6a thus made thin naturally exhibits an inferior strength. A second problem arises when the flange portion 14 is folded so that the peripheral portion 6 U letter shaped in section is formed at the steps shown in FIGS. 6 to 8. More specifically, at the steps shown in FIGS. 6 and 7 only a natural bent portion is formed by folding the flange portion 14 and no work hardening is not performed. The FIG. 8 step is then carried out in such situation and the outer peripheral side 6a of the peripheral portion 6 U letter shaped in section is brought to be close to the inner peripheral side 6b so that the spacing may be decreased, as shown in FIG. 9. As a result, the bent portion 6c becomes thinner and weak.
For the above described reasons, a sufficient strength of the outer peripheral side 6a of the peripheral portion 6 enough to withstand the pressure caused in a curling process was not attained.